The present invention relates to an active matrix type liquid crystal display device integrated with a driving circuit for displaying images according to inputted digital image signals, and more particularly to a liquid crystal display device that is little affected by non-uniform properties of elements and that is capable of reducing the power consumption and the cost by a large amount by digitally driving all circuits.
An active matrix type liquid crystal display device will be taken as an example to illustrate a conventional liquid crystal display device.
As shown in FIG. 24, the liquid crystal display device is composed of a pixel array (ARY), a scanning signal line driving circuit (GD) and a data signal line driving circuit (SD).
The pixel array ARY includes a plurality of scanning signal lines (GL1, GL2 . . . ; the signal lines will be inclusively denoted as GLs) and a plurality of data signal lines (SL1, SL2, . . . ; the signal lines will be inclusively denoted as SLs) which cross with each other. A pixel (PIX) is provided in each region enclosed by two adjacent scanning signal lines GLs and two adjacent data signal lines SLs to form a matrix as a whole.
The data signal line driving circuit (SD), which is synchronized with a timing signal such as a clock signal (CKS), samples an inputted image signal (DAT), amplifies the image signal DAT as necessary, and writes the image signal DAT into the data signal line SL. The scanning signal line driving circuit GD, which is synchronized with a timing signal such as a clock signal (CKG), sequentially selects one of the scanning signal lines GLs, writes into the pixel PIX an image signal (image data) that has been written into the data signal line SL, and lets the pixel PIX retain the image data written into the pixel PIX, by controlling opening and closing of a switching element in the pixel PIX.
As shown in FIG. 25, the pixel PIX includes a pixel transistor (SW), as a switching element, composed of a field effect transistor and a pixel capacity composed of a liquid crystal capacity (CL) and a supplementary capacity (CS) that is added if necessary.
The data signal line SL is connected to one of two electrodes of the pixel capacity via the drain and source of the pixel transistor SW. The gate of the pixel transistor SW is connected to the scanning signal line GL. The other electrode of the pixel capacity is connected to a common electrode line which is common to all the pixels. The voltage applied to the liquid crystal capacity CL modulates the transmittance or reflectance of the liquid crystal, thereby enabling the liquid crystal to act as a display.
Incidentally, as to the conventional active matrix type liquid crystal display device, an amorphous silicon thin film formed on a transparent substrate such as glass is used as a material of the substrate of the pixel transistor SW, and ICs are externally provided to function as the scanning signal line driving circuit GD and the data signal line driving circuit SD.
Meanwhile we have seen in recent years the development of a technique for forming a pixel array and driving circuit in a monolithic manner by using a polycrystal silicon thin film, in response to various needs, for example, to improve the driving capabilities of pixel transistors for the realization of a big display screen, to cut down on the mounting costs of driving ICs, and to improve reliability in mounting. In pursuit of the realization of an even larger display screen and even lower costs, attempts are being made to form a transistor from a polycrystal silicon thin film on a glass substrate at a processing temperature below the distortion point (about 600xc2x0 C.) of glass.
Such a liquid crystal display device integrated with a driving circuit has a configuration including, for example, a scanning signal line driving circuit (GD), a data signal line driving circuit (SD) and an pixel array (ARY) composed of pixels (PIXes) arranged in a matrix form, the circuits and pixels being provided on an insulating substrate (SUB), as shown in FIG. 26. The scanning signal line driving circuit GD and the data signal line driving circuit SD are connected to a control circuit (CTL) and a voltage generating circuit (VGEN).
A method for writing image data into a data signal line will be explained next. There are two methods for driving a data signal line: analogue and digital. When an IC is externally provided, the external IC incorporates an amplifier circuit to secure a driving capability in either of the methods. However, the liquid crystal display device integrated with a driving circuit employs as composing elements polycrystal silicon thin film transistors, whose properties are non-uniform. If an analogue circuit such as an amplifier circuit is used, the non-uniform properties result in non-uniformity in the output voltage, which in turn causes vertical stripes to appear on the displayed image. Therefore, generally, a driving circuit with no internally provided amplifier circuit is employed in the liquid crystal display device integrated with a driving circuit.
The following will explain a point-to-point successive driving method, which is most typically used in the liquid crystal display device integrated with a driving circuit, as an example of the analogue method.
The point-to-point successive driving method includes a data signal line driving circuit composed of a scanning circuit (shift register; SR), a buffer circuit (BUF), and sampling circuits (SMPs) corresponding to respective colors of red (R), green (G) and blue (B), as shown in FIG. 27. An image signal (DAT) inputted into an image signal line is written into a data signal line (SL) by opening and closing the sampling circuits SMPs in synchronization with an output pulse of each stage of the scanning circuit SR. The buffer circuit BUF latches in and amplifies an output signal out of the scanning circuit SR, and generates an inverse signal as necessary.
The method has an advantage of a very simple circuit arrangement, but has several disadvantages as well: Since the image signal DAT needs to be written into the data signal line SL in a short period of time (one dot period or approximately several times the dot period), the output impedance of an external circuit for supplying the image signal DAT should be low. Also, if the source of the image signal DAT is a digital signal, the image signal DAT needs to be converted to an analogue signal. Therefore, the total power consumption of the liquid crystal display device, including the power consumption at an external image signal generating section, becomes very large.
FIG. 28 shows a configuration example of the system. Since the image signal DAT is inputted to the data signal line driving circuit SD, the configuration needs a digital/analogue converter (DAC) and a buffer amplifier (AMP), consuming a very large amount of power.
Various configurations are possible for the data signal line driving circuit of a digital method. Here will be explained a multiplexer method for selecting one of externally supplied gray scale voltages and supplying the selected gray scale voltage to a data signal line directly (without amplifying). The following example illustrates a case in which the input image signal is of three bits (eight gray scales) with respect to each color of R, G, and B.
The data signal line driving circuit includes, as shown in FIG. 29, a scanning circuit (shift register; SR), nine (=3 bitsxc3x97RGB) latch-in circuits (LATs), as many transfer circuits (TRFs) as the latch-in circuits LATs, three decoder circuits (DECs) respectively composed of eight (=23) AND circuits, and twenty-four (=23xc3x97RGB) analogue switches (ASWs).
To this data signal line driving circuit are supplied a clock signal (CKS), a start signal (SPS), a transfer signal (TRP), nine (=3 bitsxc3x97RGB) digital image signals (SIGs), and eight (=23) gray scale power supplies (VGSes). The data signal line driving circuit latches in the digital image signal SIG by opening and closing the latch-in circuit LAT in synchronization with an output pulse of each stage of the scanning circuit SR, and transfers the digital image signal SIG to the decoder circuit DEC with the transfer circuit TRF during a horizontal blanking period. Then the data signal line driving circuit selects one of the eight gray scale power supplies according to a signal decoded by the decoder circuits DECs to output the selected gray scale power supply to the data signal line SL during a next horizontal scanning period.
According to this method, the image signal can take almost as much time as one horizontal period to be written into the data signal line SL, and is therefore not likely to be written improperly. However, as described above, the method has disadvantages: The circuit becomes very large (even a driving circuit of three bits input needs nine latch-in circuits, nine transfer circuits, twenty four AND circuits and twenty-four analogue switches), and requires as many external power supply circuits of a low output impedance (gray scale power supplies VGSes) as the display gray scales, the external power supply circuits being capable of directly writing the gray scale voltages into the data signal line SL.
FIG. 30 shows a configuration example of this system. The system needs to lower the output impedance of a gray scale voltage generating circuit (VGN) for supplying the gray scale voltages in the same manner as in the previous system. In other words, the system needs a driving capability to output the same gray scale voltages to all the data signal lines. Therefore, it is predicted that the total power consumption of the system becomes very large.
Incidentally, portable information terminals have become very popular in recent years. Those devices are in most cases driven by a battery, which strongly requires the display device incorporated therein to be power-saving as well as to be portable (small). However, the conventional driving method as explained above results in a very large power consumption outside the display device, for example, at an external power supply circuit, creating a possible obstacle in realizing a power-saving character with the system.
For this reason, to further reduce the power consumption, such a configuration and driving method should be adopted that the power consumption is reduced in parts other than the display device (pixel array and driving circuit). That is, a driving method using no analogue circuit and intermediate voltage generating circuit (gray scale voltage generating circuit) which may consume a large amount of power is preferred.
In addition, a configuration and driving method that can realize a reduction in size of the display device as well as the above mentioned reduction in power consumption is more preferred.
The backlight accounts for more than half the total power consumption in a typical color liquid crystal display device. A reflection type display device with no backlight has been developed as a result of giving a priority to reduction in power consumption. The reflection type liquid crystal display device does not have a very high contrast ratio, and in some cases does not need to adopt a conventional driving method. To put it in a different manner, the reflection type liquid crystal display device still has room for incorporation of a cheaper driving method and a driving method that can realize the reduction in power consumption.
As for the reduction in size of the device, as already mentioned, a liquid crystal display device composed of a driving circuit and pixel array integrally formed by using, for example, polycrystal silicon thin film transistors is very useful. However, such a liquid crystal display device has the following disadvantages.
As with the polycrystal silicon thin film transistor, the silicon crystals have diameters of the same order as the size of elements such as a thin film transistor and a resistor made of a polycrystal silicon thin film. The polycrystal silicon thin film transistor therefore inevitably has less uniform properties than do elements on a monocrystal silicon substrate. If such an element is used to configure the above mentioned analogue driving circuit or multiplexer digital driving circuit, the gray scale voltages for display may not be written with high precision mainly because of the non-uniform properties of the transistors of the output stage, failing to perform proper gray scale display.
Besides, especially when the polycrystal silicon thin film transistors is formed at a relatively low temperature below 600xc2x0 C., the size of elements becomes large because of contraints in the driving capabilities and the withstand voltages of the elements, allowing the non-uniform properties to affect more greatly.
Consequently, preferably, the liquid crystal display device integrated with a driving circuit using polycrystal silicon thin film transistors employs a configuration and driving method that is not affected by the non-uniform properties of the elements, i.e., that does not require a highly precise write-in voltage.
An object of the present invention is to offer an active matrix type liquid crystal display device integrated with a driving circuit composed of polycrystal silicon thin film transistors that is little affected by non-uniform properties of elements and that consumes much less power.
In order to accomplish the object, an active matrix type liquid crystal display device in accordance with the present invention is characterized in that it includes:
a plurality of data signal lines arranged in one direction;
a plurality of scanning signal lines arranged in a direction crossing the plurality of data signal lines;
a plurality of pixels provided in a matrix form and connected to the plurality of data signal lines and to the plurality of scanning signal lines;
a data signal line driving circuit for supplying image data to the plurality of data signal lines; and
a scanning signal line driving circuit for supplying a scanning signal to the plurality of scanning signal lines,
wherein the data signal line driving circuit and the scanning signal line driving circuit are composed of polycrystal silicon thin film transistors that are formed on the same substrate as are the plurality of pixels,
wherein each of the plurality of pixels is composed of a plurality of subpixels, and the plurality of subpixels are driven by a binary display.
In the above configuration, the active matrix type liquid crystal display device integrated with a driving circuit carries out half-tone display with an area gray scale display method, according to which a pixel is composed of a plurality of subpixels and the area of display regions is changed by an image signal corresponding to a binary display. Therefore, the configuration eliminates the needs to externally input an analogue signal and an intermediate voltage, and enables the driving circuit to be configured only with digital circuits.
This can reduce the power consumption of the system by a large amount. Especially, when such a liquid crystal display device is used in a reflection mode of a relatively small contrast ratio, it exhibits a high manufacturing yield, superior display uniformity, and very low power consumption.
Also, the driving circuit, being composed of only digital circuits, can tolerate non-uniform properties of elements to some extent and prevent a fall in its non-defective ratio.
Adopting the area gray scale display method causes the data signal lines to increase in number, and it is therefore anticipated that the cost of the driving circuit and the mounting cost of the driving circuit might increase. Nevertheless, the above configuration, in which the driving circuit is integrated, can achieve a large reduction in cost, compared to other display methods.
In addition, since the non-uniform properties of elements can be tolerated as a result of the adoption of the area gray scale display method, the polycrystal silicon thin film transistors can be formed on the substrate by processing in which the highest temperature does not exceed 600xc2x0 C.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.